Cold Hands and Shy Hermit Crabs

A Morning at Gull Point

1Yusra Kamau's sixth-grade class spent a morning at the Gull Point marine field station, where rocky tide pools fill with seawater at low tide. Their guide, marine biologist Desmond Achterberg, started with the students' own senses. He explained that the skin holds temperature receptors that detect warming and cooling, as well as touch receptors. Signals from these receptors travel along nerves to the brain.

2First, each student held one hand in a tide pool for two minutes. The water was 12 °C the whole time. Every 30 seconds, each student rated how cold the water felt, from 1 (not cold at all) to 10 (very cold), and the class found the mean rating.

3Next came the three-bucket test. Mr. Achterberg had filled one bucket with cold seawater at 12 °C, one with seawater that had warmed in the sun to about 30 °C, and a middle bucket with a mix at 21 °C. Each student held the left hand in the cold bucket and the right hand in the warm bucket for one minute. Then the student put both hands into the middle bucket at once and said how the water felt to each hand.

4Last, the class visited the station's hermit crab tank. Hermit crabs live inside empty snail shells and pull back into them when danger comes near. Their eyes have light-sensing cells that detect shadows, such as the shadow of a diving gull. Mr. Achterberg slowly passed a flat paddle over the tank so its shadow crossed 10 crabs, then waited one minute before the next pass. He never touched the crabs. The class counted how many crabs pulled into their shells on each pass. Right after the sixth pass, he passed a wide round lid over the tank instead, and its shadow made 8 of the 10 crabs pull in at once. The next morning, the station staff ran one more shadow pass and sent the class the result.